A test apparatus for gravimetric testing of multi-channel pipettes is known from the published document EP 2 759 816 B1. Such test apparatuses contain a plurality of load cells that are offset from one another in a housing. The load receptors of said load cells are arranged equidistant in a straight, horizontal row. Each load receptor carries a narrow, cylindrical vessel, into which liquid can be pipetted from a pipette during the test procedure. The arrangement of the individual load receptors and their pipetting vessels is adapted to the arrangement of the pipette tips of the multi-channel pipette to be calibrated. In this way, all of the pipette channels can be emptied simultaneously into the respectively assigned pipetting vessels; and the respective pipetting volume can be determined gravimetrically. Before each new pipetting process, the load cells can be tared as needed, so that the respective pre-fill level of the vessels does not affect the individual measurements. However, after a certain number of test procedures, it is absolutely necessary to empty the pipetting vessels. For this purpose the cited published document discloses the parallel emptying of all of the pipetting vessels by suction using a multi-channel suction pump.
Such suction pumps typically comprise a single pumping station, which generates the necessary negative pressure and which is connected to the pump port of a distributor head conforming to its genre. The distributor head comprises a plurality of hollow needles, which are fixed to the housing of the distributor head; and said distributor head has an arrangement and orientation that are adapted to those of the pipetting vessels of the gravimetric test apparatus. The individual hollow needles are connected in a liquid tight manner to the pump port. For this purpose there is usually a main line that is connected to the pump port, from which individual branch lines diverge to the individual hollow needles. Of course, it is also possible to route the individual line from each hollow needle to the common pump port. For the pumping out operation, the distributor head is positioned in such a way that all of the hollow needles are immersed in the pipetting vessels as far as to the desired immersion depth. Then the negative pressure, generated by the pumping station, is distributed to all of the suction channels, so that the liquid is aspirated from all of the pipetting vessels in parallel.
However, problems may arise, if the fill level in one pipetting vessel is significantly lower than in another pipetting vessel. The vessel that is only slightly filled will soon be sucked dry, so that the corresponding hollow needle draws air. If an extremely strong and, thus, large, heavy and expensive pumping station is not used, the result will be a disruption in the liquid column in the common line area, so that the aspiration will stop altogether. Therefore, vessels that are filled to a higher level will not be completely aspirated. If, however, it is falsely assumed, as the pipetting procedure continues, that all of the vessels are empty, then the vessels may overflow and cause damage to the sensitive inner structure of the test apparatus.